By Tom Verducci He’s a homegrown slugger with a marketable brand all his own. He and New York soon will find out how much that’s worth. Welcome to The Opener, where every weekday morning during the regular season you’ll get a fresh, topical column to start your day from one of SI.com’s MLB writers. During the lockout, when players fought for more money for young players, a veteran front office executive told me they already owned a tool to achieve that very goal. “More five-year superstar players should go to hearing [in arbitration] at a high number,” the source said. “Even if you win a low p…